An oxidation-reduction reaction that is also a synthesis reaction

a. 2mg(s) + o2(g) --> 2 mgo(s)
b. pb^2+(aq) + cro4^2-(> pbcro4(s)

, this is the redox reaction and also a synthesis reaction.

A redox reaction is that where reduction and oxidation take simultaneously.  

The synthesis reaction is one in which the product is formed.

(A) The reaction of Mg with will form MgO.

Since there is the formation of the product by reaction of 2 reagents, it is a synthesis reaction.

In this reaction, Mg is getting oxidized and oxygen is getting reduced.

So this is a redox reaction as well.

(B) The reaction of Pb with chromate will result in .

There is a formation of product in the reaction, so it is a synthesis reaction.

There is no change in the oxidation state of lead, so it is not a redox reaction.

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The oxidation reduction reaction which is also a synthesis reaction is:

A..

Explanation:

A Synthesis reaction is a reaction in which two or more reactants combine to form a single product.

A redox reaction(oxidation reduction) is a reaction in which one species is oxidised and one is reduced simultaneously.

Oxidation: Loss of electron or increase in oxidation state

Reduction: Gain of electron or decrease in oxidation state

In both cases A,B two reactant are combining to give a single compound.So both are synthesis reaction.

A. is a redox reaction :

Oxidation of Mg :

(increase in oxidation state of Mg from 0 to +2)

Reduction of Oxygen:

(decrease in oxidation state of O from 0 to -2)

on combining above half-reactions:

Its both redox and synthesis reaction.

But there is no change in oxidation states of elements in Reaction B

oxidation state of Pb is (+2) in both and :

Cr has (+6) oxidation state in both and  :

No change in oxidation state so no loss/gain of electrons .Hence not a redox reaction

B. is not a oxidation reduction reaction ,its just a synthesis reaction.

Hence,

The oxidation reduction reaction which is also a synthesis reaction is:

A..

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explanation: